Irnmunoglobulin G (IgG) from patients with heparin-induced thrombocytopenia binds to platelets in an environment that contains approximately equimolar concentrates of platelet factor 4 (PF4) and heparin. The binding requires the F(abl)2 end of the IgG but not the F end. The specific IgG binding to platelets can be isolated by binding to PF4 that is attached to heparin-sepharose. This study has been concluded and the results published (Journal of Laboratory and Clinical Medicine 1996; 127:435442).